A (Dark) Matter of Time

Physicists get creative in their search for dark matter particles

Cosmological experiments suggest dark matter really is out there. Data collected with the Hubble Space Telescope is helping astronomers map dark matter in space along with X-ray pictures of colliding galaxies, measurements of cosmic background radiation, and analysis of the way stars on the ends of galactic arms rotate. Physicists think that dark matter provides a gravitational structure for the Universe, influencing—but not usually interacting with—the matter we perceive. But dark matter isn’t just “out there”—we’re searching for it here on Earth as well.

Most dark matter labs are deep underground, minimizing background signals from cosmic radiation. Deep in these dark chambers,modern-day prospectors are coming up with creative ways to detect interactionsbetween dark and known matter, exploiting solids, liquids, and gases. “The race is on [to find the first dark matter particles],” says Florida State University theoretic physicist Howard Baer, a member of the U.S. Dark Matter Scientific Assessment Group.The stakes are high. Baer predicts a Nobel Prize for the winner.

Though far from established, the most likely dark matter candidates are huge particles called weakly interacting massive particles (WIMPs) or much lighter axion particles. As of yet, no one has been able to detect either—and “no one’s sure they exist,” explains Baer. Here are six schemes that researchers are taking on to find the illusive dark stuff:

Solid Efforts
At a former iron mine in northeast Minnesota, the Cryogenic Dark Matter Search (CDMS) team is looking for WIMP collisions with solids like germanium, a material with atoms roughly the same size as predicted WIMPs. Stacks of ultra-cooled germanium detector “crystals”—about the size and shape of hockeypucks—sit in the old mine, shielded from cosmic radiation by about half a mileof Earth. Each crystal has more than a thousand small thermometers on one side and electrodes on the other.

WIMP-germanium collisions should create heat-producing phonons—very delicate sound waves—in the crystal, creating detectable heat and changing the crystal from a superconducter (without electrical resistance) to amore natural system (with electrical resistance). WIMPs should also shove germanium nuclei out of position, knocking electrons into one another. An electrode on the flip side of the “hockey puck” should detect electric fields started by WIMPs roughing up germanium.

Ultimately, the team plans to “go deeper,” according to Priscilla Cushman, a CDMS physicist at the University of Minnesota, and set up complimentary experiments at a lab in Canada (aka, “SNOLAB”), located nearly two miles underground. At these depths there are even fewer cosmic rays capable of producing WIMP-mimicking neutrons. No word on when the CDMS team plans to drop the pucks at SNOLAB.

Noble Pursuits
The CDMS group aren’t the only ones who’d like to see WIMPs push nuclei around. Their competitors in Europe, including the XENON teamworking at the Gran Sasso underground lab near Rome, Italy, have forgone solids in favor of liquids. They’re looking for WIMP activity within vats of dense noble liquids such xenon and argon, elements in the helium family.

The advantage: noble liquid are cheaper and easier to setup, according to Baer. The idea is that WIMPs shooting through the heavy atoms will eventually knock around nuclei, exciting the atom and giving off light energy that can be absorbed by phototubes flanking the tank as well as electronic activity that can be detected by detector wires in the liquid.

A few groups are also experimenting with high-density, pressurized noble gases, though these experiments quite preliminary.

Toil and Trouble
Though the projects are still in their early stages, groups like Fermilab’s Chicagoland Observatory for Underground Particle Physics (COUPP) are investigating bubble chamber technology as yet another WIMP search tool. This involves superheating liquids—often fluorocarbons (though a numberof liquids are being tested)—above their boiling point at a controlled temperature at which they no longer bubble.

“The superheated liquid wants to boil, but it’s held above its boiling point,” Cushman explains. At the right temperature and pressure, electrons and photons shouldn’t have enough mass to create bubbles. WIMPs will. If WIMPs shoot through the scorching liquid, their mass should start the liquidbubbling, revealing dark matter.

Lights, Camera, Axion!
While there are lots of WIMP hunters, only one group is seriously looking for axions, the other main dark matter candidate. Like WIMPs, physicists predict that axions will interact very weakly with other matter. But researchers might be able to see them in a microwave-magnetic field with very sensitive electronics. At least, that’s what the Axion Dark Matter Experiment (ADMX) is betting on. They’re trying to find the right conditions for converting axions into light waves at a lab in California. This all happens in a super-cooled cylinder too, to prevent normal vibrations from getting in theway.

Smashy-smashy
Switching on in Switzerland next year with an impressive 13 trillion electron volts of energy, all aimed at orchestrating atom collisions, the Large Hadron Collider (LHC) will potentially recreate conditions similar to those following the Big Bang. Theorists predict that all this smashing could create dark matter and more: a whole cornucopia of particles that make theoretical physicists’ hearts pitter-patter. The LHC “could turn into a dark matter factory,” Baer explains.

Ice, ice... maybe?
Not all terrestrial efforts are focused on direct dark matter detection. Some, like the IceCube Neutrino Detector at the South Pole, are trying to catch dark matter products—like neutrinos (itty-bitty, highenergy particles)—on the move. The dark matter connection? One route to neutrinos may be WIMP annihilation in the Sun or Earth’s core. Because they’re so small, catching neutrinos is tricky but probably not impossible. Occasional collisions between neutrinos and ice create a particle called a muon that gives off a slight blue light.

And there’s plenty of ice at the South Pole. Scientists there are turning an entire cubic kilometer of ice into a giant neutrino “telescope” by drilling thousands of holes deep into the clear ice and planting round photomultiplier tube sensors to capture light from ice-neutrino fender-benders. IceCube is scheduled for completion by 2011.